SOLUTION: I am quested with this one f(x)=5x+4; g(x)=7x-1 and solve for (f*g)(3) I got 518 but that wasn't right this is what I had35x�+23x-4 and when I put in 3 I got 518 but that was

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Question 1016141: I am quested with this one
f(x)=5x+4; g(x)=7x-1 and solve for (f*g)(3) I got 518 but that wasn't right
this is what I had35x�+23x-4 and when I put in 3 I got 518 but that was wrong.
Found 3 solutions by Theo, Alan3354, MathTherapy:
Answer by Theo(13342)   (Show Source):
You can put this solution on YOUR website!
f(x) = 5x + 4
g(x) = 7x - 1

(f*g)(x) = f(x) * g(x).

that should get you (5x+4) * (7x-1) = 35x^2 -5x + 28x - 4 which is equal to:

35x^2 + 23x - 4

replace x with 3 and you get 35 * 3^2 + 23 * 3 - 4 = 380.

f(3) = 5*3 + 4 = 19
g(3) = 7*3 - 1 = 20
19 * 20 = 380.

not sure where you got 580 from.

it should be 380.

i think i know where you went wrong.

35 * 3^2 = 35 * 9 = 315
23 * 9 = 207
-4 = -4

315 + 207 - 4 = 518

it's the middle term.
you multiplied by 9 when you should have multiplied by 3.

Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website!
f(x)=5x+4; g(x)=7x-1 and solve for (f*g)(3) I got 518 but that wasn't right
this is what I had35x�+23x-4 and when I put in 3 I got 518 but that was wrong.
-----------
It's simpler to sub 3 for x first.
f(x)=5x+4; g(x)=7x-1
f(3) = 19
g(3) = 20
--> 19*20 = 380

Answer by MathTherapy(10549)   (Show Source):
You can put this solution on YOUR website!

I am quested with this one
f(x)=5x+4; g(x)=7x-1 and solve for (f*g)(3) I got 518 but that wasn't right
this is what I had35x�+23x-4 and when I put in 3 I got 518 but that was wrong.

(f*g)(3) = f(3) * g(3)
f(x) = 5x + 4
f(3) = 5(3) + 4
f(3) = 15 + 4, or 19
g(x) = 7x - 1
g(3) = 7(3) - 1
g(3) = 21 - 1, or 20
(f*g)(3) = f(3) * g(3) = 19(20), or
OR
It can also be done the way you did it, but I wouldn't, because the above is easier.
Anyway, how did you come up with 518 when 3 was substituted for x?